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Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

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628 NITRONATES<br />

by more than 1.5 ppm, whereas the signal for 13 C is shifted by approximately<br />

30 ppm). These chemical shifts are similar to those obta<strong>in</strong>ed earlier for cations<br />

generated by silylation of nitrones (479).<br />

Seven of the observed cations (349) are derivatives of six-membered cyclic<br />

nitronates. The predom<strong>in</strong>ant conformations of the start<strong>in</strong>g nitronate <strong>and</strong> the correspond<strong>in</strong>g<br />

cation are identical for almost all of these species, the NMR spectra<br />

show<strong>in</strong>g only one set of signals for each nitronate <strong>and</strong> the derived cation. By<br />

contrast, silylation of nitronate (348a) gives rise to two conformers of cation<br />

(349a) <strong>and</strong> (349a ′ ), which are observed separately <strong>and</strong>, consequently, have an<br />

abnormally high barrier of the r<strong>in</strong>g <strong>in</strong>version.<br />

For four cations (349b–e), the thermodynamic parameters of the equilibrium<br />

(Table 3.21) <strong>and</strong> the exponents n <strong>in</strong> the equation of the equilibrium constant were<br />

determ<strong>in</strong>ed.<br />

These data show that the im<strong>in</strong>ium cations exist <strong>in</strong> dichloromethane as ion<br />

pairs. Apparently, this is the reason why the r<strong>in</strong>g <strong>in</strong>version <strong>in</strong> cation (349a)ish<strong>in</strong>dered,<br />

tak<strong>in</strong>g <strong>in</strong>to account that the r<strong>in</strong>g <strong>in</strong>version requires the energy-consum<strong>in</strong>g<br />

cleavage of the ion pair.<br />

Several facts, which are of importance for the use of cations (349) <strong>in</strong> C,Ccoupl<strong>in</strong>g<br />

reactions, follow from the existence of these cations as ion pairs.<br />

First, a decrease <strong>in</strong> the concentration of solutions would lead to a shift of the<br />

equilibrium (348⇆349) toward <strong>in</strong>itial nitronates (348). Second, high negative<br />

ΔS ◦ of the equilibrium demonstrate that an <strong>in</strong>crease <strong>in</strong> the temperature also would<br />

shift the equilibrium (348⇆349) toward nitronates. Hence, it is advantageous to<br />

perform the C,C-coupl<strong>in</strong>g reactions of cations (349) at low temperatures <strong>and</strong> at<br />

high concentrations of the molecules <strong>in</strong>volved <strong>in</strong> the reaction.<br />

As can be seen from Scheme 3.204, it is necessary to use powerful LA as<br />

catalysts for the generation of cationic <strong>in</strong>termediates (349). However, similar<br />

acids are hydrolytically very unstable.<br />

Table 3.21 Thermodynamic parameters of equilibrium (348) + SiOTf⇄(349)<br />

Range Δ G0 Equilibrium n (averaged) of T, K<br />

293,<br />

kJ/mol ΔH0 , kJ/mol ΔS 0 , J/mol •K<br />

348b→ ← 349b 0.98 ± 0.05 183 ÷ 207 18.0 ± 1.1 − 35.5 ± 0.4 −182.7 ± 2.4<br />

348c → ← 349c 1.04 ± 0.08 190 ÷ 220 1.7 ± 3.3 − 29.0 ± 1.3 −104.8 ± 6.7<br />

348d→ ← 349d 1.04 ± 0.06 180 ÷ 210 12.2 ± 2.3 − 26.4 ± 0.9 −131.7 ± 4.8<br />

348e → ← 349e 0.84 ± 0.05 210 ÷ 240 9.3 ± 2.3 − 59.5 ± 1.0 −234.9 ± 4.3<br />

O<br />

O<br />

N<br />

348b<br />

OCOC6H4-NO2<br />

Ph<br />

O N<br />

348c<br />

Me<br />

O<br />

K = [349]n<br />

(1)<br />

[348] [SiOTf]<br />

OEt<br />

N<br />

Ph<br />

O<br />

348d<br />

Me<br />

N<br />

348e<br />

O<br />

OSiMe2Bu t

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